U.S. patent application number 12/678772 was filed with the patent office on 2010-09-30 for nh3-monitoring of an scr catalytic converter.
This patent application is currently assigned to FEV Motorentechnik GmbH. Invention is credited to Bastian Holderbaum.
Application Number | 20100242454 12/678772 |
Document ID | / |
Family ID | 39493905 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100242454 |
Kind Code |
A1 |
Holderbaum; Bastian |
September 30, 2010 |
NH3-MONITORING OF AN SCR CATALYTIC CONVERTER
Abstract
The present invention relates to an internal combustion engine
(1) with an SCR catalytic converter (11) and with a condition
monitor (10) of the NH3 level of the SCR catalytic converter,
wherein the condition monitor is connected to a first (14) and a
second detecting module (15), each of which determines the NH3
level in a different way. In addition, a method for the
determination of the NH3 level of an SCR catalytic converter is
claimed.
Inventors: |
Holderbaum; Bastian;
(Aachen, DE) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Assignee: |
FEV Motorentechnik GmbH
Aachen
DE
|
Family ID: |
39493905 |
Appl. No.: |
12/678772 |
Filed: |
September 18, 2007 |
PCT Filed: |
September 18, 2007 |
PCT NO: |
PCT/EP07/08115 |
371 Date: |
May 19, 2010 |
Current U.S.
Class: |
60/301 ;
73/114.75 |
Current CPC
Class: |
F01N 2550/03 20130101;
B01D 53/9409 20130101; F01N 3/208 20130101; F01N 2560/021 20130101;
F01N 2560/026 20130101; F01N 2900/1622 20130101; F01N 2900/1814
20130101; Y02T 10/40 20130101; Y02T 10/12 20130101; F02D 41/0275
20130101; F01N 2610/02 20130101; Y02T 10/24 20130101; F01N
2900/1402 20130101; B01D 53/9495 20130101; F01N 9/005 20130101;
F01N 11/005 20130101; Y02T 10/47 20130101; B01D 2251/206
20130101 |
Class at
Publication: |
60/301 ;
73/114.75 |
International
Class: |
F01N 11/00 20060101
F01N011/00; F01N 3/10 20060101 F01N003/10; G01M 15/04 20060101
G01M015/04 |
Claims
1. An internal combustion engine comprising: at least one SCR
catalytic converter; and at least one condition monitor of SCR
catalytic converter for its NH3 level, wherein the condition
monitor is connected to at least one first and one second detecting
module that determine the NH3 level in different manners.
2. The internal combustion engine according to claim 1, further
comprising a correlation unit connected to the first and second
detecting module.
3. The internal combustion engine according to claim 1, wherein a
stored weighting function by means of which an NH3 slip can be at
least partially compensated between different detections of the NH3
level.
4. The internal combustion engine according to claim 1, wherein at
least one of the first and second detecting module includes a
sensor that is capable of recording a value in relation to the NH3
level.
5. The internal combustion engine according to claim 1, wherein at
least the first and/or the second detecting module includes an
integration of a mass flow relative to a supplied and consumed NH3
mass flow and/or has one or more stored characteristic diagrams
containing a dependence of an NOx conversion on a stored NH3 amount
in SCR catalytic converter and/or a physical model of SCR catalytic
converter that has kinetic approaches to a storage behavior and/or
a characteristic diagram-based determination of a current NH3 level
of SCR catalytic converter.
6. The internal combustion engine according to claim 1, wherein the
condition monitor is coupled to a load check and/or an SCR
temperature check, wherein an NH3 slip avoidance threshold is
present, and if the threshold is exceeded, an operating mode
changeover of the internal combustion engine is initiated.
7. The internal combustion engine according to claim 1, wherein the
condition monitor is coupled to an NH3 level regulation.
8. A method for determining an NH3 level of an SCR catalytic
converter for an internal combustion engine according to claim 1,
said method comprising the steps of: determining a value relevant
to a respective NH3 level by at least two different determination
paths, and wherein the at least two different determination paths
are correlated to deduce a resulting NH3 level.
9. The method according to claim 8, wherein a respective NH3 level
is acquired by different determination paths, and wherein the
different determination paths are correlated with one another in
order to arrive at a resulting NH3 level.
10. The method according to claim 8, wherein a drift between at
least two differently determined values is deduced from the results
of different determination paths.
11. The method according to claim 8, wherein a diagnostic system
can be created that uses different determination paths to check a
subsystem for determining the NH3 level.
12. The method according to claim 8, wherein a threshold value is
set for a beginning of an NH3 slip, and if the threshold is
exceeded, the internal combustion engine changes its operating
mode.
13. Application of at least one of the determination paths
according to claim 8 for monitoring an SCR catalytic converter of
an internal combustion engine.
14. An internal combustion engine comprising: a SCR catalytic
converter; a condition monitor for monitoring a NH3 level of said
SCR catalytic converter; a first detecting module connected to said
condition monitor; and a second detecting module connected to said
condition monitor; wherein said first detecting module determines
said NH3 level of said SCR catalytic converter in a first manner,
and wherein said second detecting module determines said NH3 level
of said catalytic converter in a second manner different from said
first manner.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. national phase of
PCT/EP2007/008115 filed Sep. 18, 2007.
FIELD OF THE INVENTION
[0002] The present invention relates to an internal combustion
engine with at least one SCR catalytic converter and a condition
motoring of the SCR catalytic converter.
BACKGROUND OF THE INVENTION
[0003] It is known from the prior art that an SCR catalytic
converter is assessed with respect to its functioning. In DE 43 15
278 A1, the monitoring of NH3 storage is discussed in general
terms, but there is no concrete information as to how an NH3 level
can be determined. It is described in DE 199 31 007 A1 that during
the storage of ammonia, certain physical properties of the SCR
catalytic converter change, which can be detected metrologically.
The applicant's unpublished WO 2007/096064 describes a regulation
and a change from a lean to a stoichiometric operation of a
four-stroke engine. In the diesel engine, on the other hand, there
is no shift to stoichiometric operation, so the regulation of the
engine operation must be done in a different manner if the exhaust
gas temperature rises sharply. It is known from paragraph 27 of EP
17 12 764 A1 that an NH3 balance is used as a method for
determining an NH3 level of the SCR catalytic converter. These
methods have the following background:
[0004] At a low exhaust gas temperature, SCR catalytic converters
have a high ability to store NH3. In addition, the effectiveness of
a catalyst increases with the storage level. An excessively high
storage level should be avoided, however since a rapid reversal of
the storage capability occurs with increasing temperature,
therefore excessive NH3 would be emitted to the environment, i.e.,
an NH3 slip, as it is referred to below, would occur. For this
reason, the storage level must be monitored and regulated to a
target value.
[0005] The problem of the present invention is to enable a reliable
and secure operating mode of an internal combustion engine with an
SCR catalytic converter in which NH3 slip can be securely
avoided.
SUMMARY OF THE INVENTION
[0006] The problem is solved by an internal combustion engine as
well as by a method disclosed herein. Advantageous configurations
and refinements follow from the respective subordinate claims.
[0007] An internal combustion engine is proposed with at least one
SCR catalytic converter and with at least one condition monitor
(10) for the NH3 level of the SCR catalytic converter, wherein the
condition monitor is connected to at least one first and one second
detecting module that determine the NH3 level in different ways.
Preferably a correlation unit is connected to the first and second
detecting modules. A refinement has a stored weighting function by
means of which an NH3 slip between different detections of the NH3
level can be at least partially compensated. At least one detecting
module preferably comprises a sensor that is capable of recording a
value in relation to the NH3 level.
[0008] It is preferred that at least the first and/or the second
detecting module comprise an integration of a mass flow relative to
a supplied and consumed NH3 mass flow and/or have one or more
stored characteristic diagrams containing a dependency of an NOx
conversion on a stored NH3 amount in the SCR catalytic converter
and/or a physical model of the SCR catalytic converter that has
kinetic approaches to a storage behavior and/or a characteristic
diagram-based determination of a current NH3 level of the SCR
catalytic converter.
[0009] Another configuration provides that the condition monitor be
coupled to a load check and/or an SCR temperature check, wherein an
NH3 slip avoidance threshold is present, and if it is exceeded, an
operating mode changeover of the internal combustion engine is
initiated.
[0010] It is additionally preferred that the condition monitoring
be coupled to an NH3 level regulation. One or more SCR catalytic
converters can be present. They can be connected in parallel and/or
in series. One or more metering functions for one or more reducing
agents can also be present. Correlation can be performed for each
individual SCR catalytic converter and/or for several SCR catalytic
converters in common.
[0011] According to another conception of the invention, a method
for determining an NH3 level of an SCR catalytic converter for an
internal combustion engine, preferably an internal combustion
engine described above or below, is proposed, in which a value
relevant to a respective NH3 level is determined by at least two
different determination paths, and they are correlated to deduce a
resulting NH3 level. Preferably a respective NH3 level is acquired
on the different determination paths, and these are correlated with
one another in order to acquire a resulting NH3 level.
[0012] A refinement provides that a drift between at least two
differently determined values be deduced from the results of the
different determination paths.
[0013] For example, a diagnostic system can be created with the
proposed method that uses the different determination paths to
check a subsystem for determining the NH3 level.
[0014] Another configuration provides that a threshold value is set
for a beginning of an NH3 slip, and if it is exceeded, the internal
combustion engine changes its operating mode. The threshold value
can be changeable, for example, more particularly, adaptable. For
example, the threshold value can be stored in a characteristic
diagram or specified by a control device.
[0015] It is further proposed that at least one of the proposed
determination paths be used for monitoring an SCR catalytic
converter of an internal combustion engine. Further characteristics
and explanations regarding the proposed internal combustion engine
and the method will be described below.
[0016] The current NH3 level is determined according to one
embodiment in at least two, preferably several ways, independently
of one another. The NH3 level of the SCR catalytic converter cannot
be directly measured. Therefore methods with which the NH3 level
can be determined must be developed or used. If NOx sensors are
used for this calculation, then it must be taken into account that
these sensors have a certain inaccuracy. Since the storage level is
derived from the integral of a difference, e.g., input NH3 amount
minus consumed NH3 amount, a considerably incorrect determination
of the storage level results over time from even small sensor
errors of a few ppm. Another advantage is therefore to achieve a
partial compensation or correction of the NOx sensor error by using
different methods for determining the NH3 level.
[0017] Moreover, an intervention in the engine control is possible
to avoid NH3 slip in case of rapidly increasing exhaust gas
temperature, so that in case of a temperature increase, higher NOx
raw emissions result simultaneously, which lead to a faster
drawdown of the stored ammonia. For example, a partial compensation
of an NOx sensor error as well as a correction of the sensor signal
or a metering can result from multiple determinations of the
storage level.
[0018] A first method contains the integration of the mass flows of
the metered NH3 as well as the NH3 consumed for NOx conversion. The
stored NH3 amount results from the difference of these two
components. In this method, the metered NH3 amount is determined
from the characteristic curve of the metering system. The converted
amount is calculated via the NOx conversion, for example, by using
NOx sensors upstream and downstream of the SCR catalytic converter,
or a model for the NOx emissions. These measurement signals or
model values are error-prone to a certain extent. Since an
integration is involved, the thus-determined value for the NH3
level becomes less accurate over time.
[0019] A second method determines the current NH3 level by way of
characteristic diagrams that contain the dependence of the NOx
conversion on the stored NH3 amount. This dependence is determined
for the SCR catalytic converter by prior experiment. The final
value of the NH3 level is determined by means of a weighting of the
partial results from the methods used. The weighting can be a
function of the various input parameters, for example, the
catalytic converter temperature or the exhaust gas mass flow.
Alternatively, the arithmetic mean can be taken.
[0020] The behavior of the first and the second methods will be
described in more detail below. The first method takes into account
the complete metered mass flow of the reducing agent. The fact that
the reducing agent must possibly first be converted to NH3 via
intermediate steps such as thermolysis or hydrolysis is ignored. In
addition, part of the reducing agent may not be available at the
SCR catalytic converter at all, due to unequal distribution or the
formation of deposits. For this reason, the NH3 level determined by
the first method is fundamentally higher than the actual NH3 level
available for NOx conversion. In contrast, the second method
directly monitors whether an NH3 level sufficient for the desired
NOx conversion is available. If the NOx conversion is lower than
desired, then the calculated level will be reduced and more
reducing agent will be metered in. However, an exclusive use of the
second method has the risk that the NOx conversion calculated by
cross-sensitive NOx sensors will continue to decline in case of an
NH3 slip, which would result in a further increase of the reducing
agent metering and thus a higher and higher NH3 slip. This can be
prevented by the simultaneous use of the first method, which
includes the absolute metered amount and thus prevents a larger and
larger increase of the metered amount.
[0021] In principle, the two methods exhibit the opposite behavior
in case of an erroneous signal of the NOx sensors. If two NOx
sensors are used for the regulation, for example, one sensor
upstream and one downstream of the SCR catalytic converter, and if
these two sensors have the same error, this will have no effect on
the regulation since only difference signals are used. In the case
of different sensor errors, on the other hand, an erroneous
determination of the NH3 level results, insofar as only one of the
above-mentioned methods is used. A combination of the first and the
second methods, on the other hand, allows a partial compensation of
the sensor error. If, for example, the downstream NOx sensor
indicates an excessively high value caused by a sensor drift or an
NH3 slip, then an excessively low NOx conversion is calculated. An
NH3 level that is higher than the actual level results in the first
method due to the integration of the difference between the metered
and the converted NH3 amounts. On the other hand, the second method
determines a lower level than actually exists. An overall more
plausible NH3 level is determined from the averaging of these
individual values, so that the regulation remains stable even in
case of a sensor error.
[0022] An excessively large deviation of the two determined levels
can also be used for adapting the NOx sensor or the metering. If
such a deviation is recognized over an applicable period of time,
then the metering is first reduced in order to check whether there
is an NH3 slip. If the deviation is not thereby reduced, then a
sensor drift can be deduced and a correction of the sensor signal
can be performed. If, on the other hand, an additional ammonia
sensor is used downstream of the SCR catalytic converter, then an
NH3 slip can be directly measured and the reduction of the metered
amount to check for an NH3 slip can be omitted.
[0023] Alongside the above-described first and second methods,
additional approaches with which the NH3 level can be determined
are possible, and their partial results can flow into the weighting
for determining the overall NH3 level.
[0024] A third method provides a physical model of the SCR
catalytic converter that models the storage behavior by means of
kinetic approaches, based on material data specific to the
catalytic converter, such as cell density, volume, specific
surface, coating material, etc. It can also be used for resetting
the monitored NH3 storage by setting the NH3 level to zero at a
high exhaust gas temperature after the lapse of an applicable time.
Such a model can be parameterized by comparison to laboratory
studies of an identical SCR catalytic converter.
[0025] A fourth method is a characteristic diagram-based
determination of the current NH3 level. In this case, the NH3 level
is determined as a function of the feed ratio, for example, the
metered NH3 concentration/NOx concentration upstream of the SCR
catalytic converter, and boundary values determining the NOx
conversion, e.g., temperature, spatial velocity, NO2/NOx ratio
upstream of the SCR catalytic converter, etc., as well as the time
constant for the storage process. Based on these values, the NH3
level can be determined by integration of the metered NH3 and NOx
amounts.
[0026] In addition, a metrological determination of the NH3 level
can be performed by utilizing the fact that physical properties of
the SCR catalytic converter change when NH3 is stored. These
connections are described in the above-mentioned patent DE 199 31
007 A1, which is incorporated in full by reference in this regard
within the scope of the present disclosure. A metrological method
for determining the NH3 level, which, in addition to the formation
of a partial result, can flow into the determination of the overall
level, is already described in DE 199 31 007 A1. The shift from
lean to lambda-1 operation is described in WO 2007/096064. But even
in a purely lean operation, a sharp increase in the load can lead
to a rise of the exhaust gas temperature and thus a reduced NH3
storage capability, so that an intervention in the engine operation
is necessary in order to be able to reduce the storage level on
time. For the possibility of how the change of operating mode can
be performed, WO 2007/096064 is incorporated in full by reference
within the scope of the present disclosure. In relation to a
possible configuration of a balancing, EP 1 712 764 A1 is
incorporated by reference.
[0027] For example, a rapid rise of the SCR catalytic converter
temperature can occur in case of a sharp increase in the load. This
has the effect that, even with metering deactivated, the amount
already stored in the SCR catalytic converter can no longer be
completely reacted in the form of NOx conversion, but can escape
into the environment as NH3 slip. This can be countered by
switching the engine into a different operating mode with higher
raw NOx emissions and possibly simultaneously lower fuel
consumption due, for example, to a reduced exhaust gas return rate
or an advanced beginning of injection.
[0028] NOx sensors have a maximum possible accuracy that may not be
sufficient for exact regulation of the metering, and moreover, they
react cross-sensitively to ammonia. Therefore it is currently
necessary to use model-based regulation systems that are difficult
to supply with data, or the metering regulation is deliberately set
up such that the maximum possible NOx efficiency is not used, in
favor of avoiding NH3 slip. The advantage of the technical teaching
described here is that at least partial compensation of measurement
errors becomes possible by using several different methods for
determining the amount of NH3 stored in the SCR catalytic
converter, wherein the influence of sensor deviations for two
methods is opposite, so that a compensation of the error is
realized, or recognition of NH3 slip or a sensor error becomes
possible. An adaptation of this sensor or the metering is thereby
possible.
[0029] In case of a sharp temperature rise of the SCR catalytic
converter, it is possible, according to another conception of the
invention, also independent, to avoid a slip of the ammonia stored
at a lower temperature by adjusting the engine operating mode in
such a way that the raw NOx emissions are elevated and the
increased NH3 conversion necessary for the reduction of these
nitrogen oxides lowers the NH3 level sufficiently quickly. Such an
engine operating mode can also lead at the same time to a lower
fuel consumption.
[0030] According to an additional conception of the invention, the
NH3 level can also be determined in other ways in addition to the
methods described above. If more than two methods are used, the
effort to supply data and the complexity of obtaining plausible
information also increase. A weighting of the individual components
can be introduced for the averaging to determine the overall NH3
level. This can also be designed to be temperature-dependent. For
example, the characteristic diagram-based level can be assigned a
higher weight in this manner at low temperatures, while the level
determined from the balance can be assigned a higher influence at
high temperatures.
[0031] Various advantages of the invention, each of which can also
be individually pursued further as an invention, independently of
the others, will be presented below: [0032] stand-alone
determination of the current NH3 storage level in several ways with
subsequent weighting and formation of an overall value; [0033] use
of a characteristic diagram that takes into account the dependence
of the NOx conversion on the level; [0034] compensation of
measurement errors occurring from sensor errors or NH3 slip by
contrary influence of this measurement error on the methods used;
[0035] checking of the deviations between the results of the
methods employed, and adaptation of the metering in case of
recognized NH3 slip or adaptation of the NOx sensor value in case
of a recognized sensor error; [0036] inclusion of additional level
determination and/or metrological detection of the level; [0037]
shift of the engine operating mode to increase the raw NOx emission
in case of a rise in the exhaust gas temperature in order to
rapidly draw down the stored ammonia to avoid NH3 slip;
[0038] A particularly preferred application of the invention, which
can also be pursued independently of the others, results as
follows, for example: [0039] regulation of the NH3 level,
particularly at low exhaust gas temperatures in the motor vehicle,
in order to achieve high NOx conversion; [0040] avoidance of a
heating strategy of the SCR catalytic converter in case an
optimized regulation leads to conformance with the NOx limit values
already at low temperature, thereby avoiding extra fuel
consumption; [0041] avoidance of an extra NH3 trap catalytic
converter downstream of the SCR catalytic converter, for example,
if the regulation is limited to an NH3 breakthrough at a minimal
level; thereby saving an additional component and thus costs and
installation space as well as additional calibration effort,
particularly in case of ODB; [0042] change of the engine operating
mode toward higher raw NOx emissions with simultaneously reduced
fuel consumption.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The invention will be described below on the basis of
illustration examples. The details and features evident from these
illustrations are not to be interpreted as limiting, however.
Rather, they are to be understood only as one of several possible
implementations or possibilities. Moreover, characteristics evident
from the individual figures can be linked with other
characteristics from other figures or from the above general
description to form additional configurations. In detail:
[0044] FIG. 1 shows a schematic representation example of the
arrangement of an internal combustion engine, an SCR catalytic
converter and additional components;
[0045] FIG. 2 shows a representation example of the dependence of
an ammonia storage capability versus the temperature of an SCR
catalytic converter,
[0046] FIG. 3 shows a representation of an NOx conversion rate as
well as an ammonia slip relative to an ammonia level of an SCR
catalytic converter,
[0047] FIG. 4 shows a schematic representation of the determination
of an NH3 level in an SCR catalytic converter in various ways and
its further processing,
[0048] FIG. 5 shows a compensation of at least two different
determination paths of an NH3 level for acquiring a level arising
therefrom,
[0049] FIG. 6 shows a representation example of regulation of an
NH3 level by means of a regulator integrated with the monitor,
and
[0050] FIG. 7 shows a contrast of various operating modes of the
internal combustion engine, wherein an NH3 slip appears in the
upper area of FIG. 7 if there is no change of the operating mode,
and the prevention of an NH3 slip by changing the operating mode is
illustrated in the lower area of FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
[0051] FIG. 1 shows a possibility of arranging various components
of the system in a representation example. This arrangement is not
to be interpreted as restrictive, however. Rather, various
components can also be arranged at different places. An internal
combustion engine 1 can be seen in FIG. 1. It is connected to an
exhaust gas system 2. A flow direction of an exhaust gas is
indicated by the arrows 3. An oxidation catalytic converter 4 is
arranged downstream of internal combustion engine 1, for example.
In place of the oxidation catalytic converter 4, there could also
be an exhaust gas return directly into internal combustion engine 1
and/or an exhaust gas turbine of an exhaust gas turbocharger. A
first NOx sensor 5 is arranged downstream of oxidation catalytic
converter 4, for example. The former is preferably arranged
upstream of an inlet of a reducing agent supply line 6 in exhaust
gas system 2. Reducing agent supply line 6 has a valve 7, for
example. A metered amount of a reducing agent can be supplied in a
controlled manner or regulated in a targeted manner by means of
this valve, for example, an injector. Valve 7 is connected for this
purpose via a data line 8 to a control device 9, for example, an
engine control device. A condition monitor 10 for an SCR catalytic
converter 11 is preferably contained in control device 9. Control
monitor 10 can also be housed, however, in a separate control or
regulation device that is connected to control device 9. For the
sake of example, at least one temperature sensor is assigned to SCR
catalytic converter 11. The temperature sensor 12 is upstream of
SCR catalytic converter 11 according to this configuration. It can
also be integrated into the SCR catalytic converter, however, or
situated downstream of it. One or more temperature sensors 12 can
also be provided at various of these sites, in order to allow
temperature monitoring of the exhaust gas stream and/or SCR
catalytic converter 11. A second NOx sensor 13 is arranged
downstream of SCR catalytic converter 11. The NOx sensors can also
be arranged in a different manner, not limited to the arrangement
presented here. In addition to condition monitor 10, the control
device 9 according to the configuration presented here additionally
implements a first detecting module 14, a second detecting module
15, a correlation unit 16 and a weighting function 17. These
individual components can preferably be situated in the same
control device but can also be present in different units
physically separated from one another. Here they are equipped with
a suitable signal transmission path such as a bus system. In order
to obtain one or more values necessary for the respective
calculation method stored in detecting modules 14, 15, they can be
connected, for example, to one or more sensors. In particular, the
result with respect to an NH3 level acquired from first detecting
module 14 and from second detecting module 15 can be correlated via
correlation unit 16. For example, it is provided that the acquired
results be adapted via a weighting function 17, so that the overall
end result is an NH3 level with which a regulation can be operated.
A regulation of the NH3 level is preferably performed by means of
an added regulator, which is likewise preferably integrated into
control device 9. A load monitor 18 is also provided. The load can
be monitored, for example, via a pedal position as illustrated.
However, the torque or the rotational speed of internal combustion
engine 1 can also be monitored for this purpose. In addition to
these illustrated components, components such as sensors,
monitoring units and/or additional catalytic converters can be
provided, however, they are not shown here in detail for reasons of
simplification.
[0052] With the internal combustion engine 1 presented here, there
is the possibility that several, preferably two, different
determination paths are used to be able to determine the level
value of SCR catalytic converter 11 more accurately. The first two
determination paths presented below are particularly suitable,
since the errors in the determination of the level compensate one
another at least in part if one determines the level from a
weighted average, for example.
[0053] The first determination path is to prepare an ammonia
balance from the amount of supplied ammonia, which is known from
the cycle time of the metering valve, and from the difference of
the NOx values upstream and downstream of the SCR catalytic
converter 11 as measured by two NOx sensors. Instead of the NOx
sensor upstream of SCR catalytic converter 11, a characteristic
diagram or a model of the NOx emissions of internal combustion
engine 1 could alternatively be used. Under the largely satisfied
condition that NOx is not stored to a great extent in SCR catalytic
converter 11, the amount of consumed ammonia can be determined from
the measured NOx difference. The remainder of the ammonia must
consequently be stored in SCR catalytic converter 11 or, in the
case of a negative balance, has been depleted. The instantaneous
level is obtained by integration of the respective stored amounts.
This balance does not take into account an ammonia slip, which
should of course be avoided with proper handling of the process. In
case of a slip, the cross-sensitivity of the downstream NOx sensor
to ammonia also comes into play. This sensor upstream of the
catalytic converter is not subjected to ammonia, since it is
situated upstream of the injection point for ammonia.
[0054] The second determination path likewise provides the
measurement of the supplied ammonia and the NOx values upstream and
downstream of the SCR catalytic converter. The storage level is not
determined in this case by integration from the NOx conversion
measured as in the first determination path; instead, the level is
determined directly as a function of NOx conversion by way of a
characteristic diagram "Ammonia level vs. NOx Conversion." The NOx
conversion is a function of the ammonia availability, in addition
to the temperature, the NO.sub.2/NO.sub.x ratio, the exhaust gas
mass flow and other boundary conditions, and thus also of the
ammonia level. This dependence is used for determining the level.
The advantage is that this method does without integration and thus
does not become more and more imprecise over time like the first
determination path. According to one configuration, this
characteristic diagram method also does not take into account an
ammonia slip or the cross-sensitivity of the second NOx sensor to
ammonia.
[0055] The crucial advantage of the combination of the two
determination paths is that errors of measurement due both to
ammonia slip and sensor errors can be recognized and partially
compensated by averaging the acquired level values. In the two
determination paths, the effect of ammonia slip and sensor errors
enter the determination paths in opposite directions. If, for
example, ammonia slip occurs, then the second NOx sensor, which is
downstream of the SCR catalytic converter, will always measure an
excessively high NOx value due to the cross-sensitivity to ammonia.
In the first determination path, an excessively low NOx and ammonia
conversion will be determined and therefore the determined ammonia
level will be too high. In the second determination path, on the
other hand, an excessively low ammonia supply will be diagnosed
from the low NOx conversion rate via the characteristic diagram and
thus the ammonia level will be too low. A plausible level value for
the ammonia can be achieved by appropriately weighted averaging.
The analysis is completely analogous for sensor errors, for
example. They also behave in opposite ways.
[0056] It may additionally be pointed out that each determination
path can itself have a correction factor or some other value with
which a deviation, a drift and/or some other change, can be
compensated. This can also be provided for the determination paths
proposed here and their respective linking with one another.
[0057] A diagnosis method for a level drift can also be performed
by at least two different determination paths. For this purpose,
for example, the supply of ammonia is reduced under otherwise fixed
operating conditions; if the levels measured by both methods drift
closer to one another as a reaction in the direction described,
then an ammonia slip should be diagnosed as a cause for the drift,
i.e., the level lies at or above the limit for ammonia slip. If the
level values drift further apart, then there is too little ammonia
in storage, caused, for example, by a sensor error. These errors
can then be compensated by correcting the sensor signal or the
metering. An analogous determination can also be made with an
increased supply of ammonia. This diagnosis can be used for
regulation, for a limit value check or as a plausibility criterion.
In this way, for example, the state of the regulation or a
threshold value can also be monitored, possibly with a subsequent
adjustment by adaptation.
[0058] For a third determination path, for example, only the input
temperatures and the ammonia and NOx quantities are required in
addition to already available characteristic parameters of the SCR
catalytic converter such as cell density, material properties and
so on, since a physical model is capable of calculating the output
values, including the storage level, on its own. This method can be
used according to an additional conception of the invention as an
additional independent method, particularly for checking the
plausibility of the combination of the first and second
determination paths, as well as being used as an individual
measuring method.
[0059] A fourth determination path treats the SCR catalytic
converter 11 as a first-order regulation timing element with
respect to the storage. For this purpose, the time constants or the
behavior over time of the ammonia storage is input into a
characteristic diagram as a function of the temperature and the
level. The ammonia level can thus be determined at any time from
the supply of ammonia and NOx, measured according to the third
determination path, for example. The timing element represents an
integration. Here, for example, the detailed physical model of the
third determination path is replaced by a black box with PT1
behavior in the storage process and DT1 behavior in the emptying of
the storage level.
[0060] FIG. 2 shows a correlation between an ammonia storage
capability, represented on the Y-axis, and a temperature of an SCR
catalytic converter, represented on the X-axis. At a low exhaust
gas temperature, SCR catalytic converters have a high ability to
store NH3 . Moreover, an efficiency of an SCR catalytic converter
increases with a storage level. An excessively high storage level
should be avoided, however, since a rapid reversal of the storage
capability occurs with increasing temperature, as shown, and
therefore excessive NH3 would be emitted to the environment. This
would result in a so-called NH3 slip. For this reason, the NH3
level of an SCR catalytic converter is monitored and, based on a
knowledge of the correlation seen in FIG. 2 specifically for an SCR
catalytic converter, a target value is preferably regulated, but is
at least initially controlled. In addition, this correlation is
used to be able to define one or more different threshold values,
for instance, for an NH3 slip.
[0061] FIG. 3 shows, in a simplified representation, a correlation
between an ammonia storage level in an SCR catalytic converter,
represented on the Y-axis [sic; X-axis], and an NOx conversion or
an NH3 slip, represented on the Y-axis. The higher the NH3 level of
the SCR catalytic converter, the greater the possibility that an
NH3 slip will occur. The amount that can escape into the
environment in case of such an NH3 slip also becomes larger with an
increasing NH3 level. Furthermore, since the storage capacity
decreases with increasing temperature but NOx emissions also
increase with rising temperature, whereas, on the other hand, the
efficiency of an SCR catalytic converter in converting NOx
increases with increasing NH3 level, it has been found, according
to another conception, which can also be further developed, that it
is advantageous to provide an intervention in a controller of the
internal combustion engine so that higher raw NOx emissions occur
in case of a rise in temperature, which lead to a faster drawdown
of the stored ammonia.
[0062] FIG. 4 shows a configuration of a possible process sequence
in an example schematic diagram. Different ways of determining the
NH3 level are used here, briefly designated as NH3 balance,
characteristic diagram and kinetics model. They can be supplemented
by additional types of determination, indicated by the empty box.
They are each provided with a weighting factor, indicated by the
weighting function 17. A temperature, a water flow or some other
parameter can serve as input parameters for a weighting. From the
totality, an NH3 level is determined, which is preferably a
component of a regulation of the NH3 level of the SCR catalytic
converter.
[0063] FIG. 5 shows a configuration example of the invention, in
which a compensation is used that is based, for instance, on types
of NH3 level determination tending to go in different directions.
Thus, the determination by way of an NH3 balance tends to move in a
different direction than the determination of the NH3 level by a
type of characteristic diagram calculation. These two correlated
methods at least reduce the otherwise existing deviation error and,
in particular, can even cancel one another out with a suitable
correlation. In case of excessive deviations, this additionally
allows a check of whether one or more of the recorded values is
possibly erroneous. In this way, an operational message can be
provided as to whether there is an error with, for example, a
sensor, a measurement unit, a correlation unit, a detecting module
or possibly an SCR catalytic converter. The possibilities for
compensation can also be different. This can take place, for
example, by weighted averaging. The respectively determined NH3
level values in particular can be at least partially compensated by
averaging.
[0064] FIG. 6 shows a configuration example of a regulation scheme
for determining an NH3 level of an SCR catalytic converter. The NH3
level is indicated as NH.sub.3Stor.sub.--.sub.act. This represents
the result of this section of the regulation method. According to
this representation, an NH3 level is determined in two ways. First,
a first NH3 level is determined via an NH3 balance. This value
NH.sub.3Stor.sub.--.sub.Balance enters into the process just like
an NH3 level determined by means of a characteristic diagram that
takes into account a dependence of an NOx conversion on the NH3
level. This value is specified as a partial result,
NH.sub.3Stor.sub.--.sub.charact. diag. With regard to the
determination by way of a balancing, for example, an integration of
a difference of the metered and converted NH3 mass flow is
performed. The determination by means of the characteristic
diagram, on the other hand, contains test results for a dependence
between the NH3 level and the NOx conversion. An NH3 level can
thereby be associated with the measured NOx conversion. This result
is corrected by an additional characteristic diagram which takes
into account the fact that the NOx conversion is a function of
additional boundary conditions such as the NO.sub.2/NO.sub.x
conversion, the NO.sub.2/NO.sub.x ratio, the spatial velocity,
etc., alongside the temperature and the NH3 level. The two partial
results are combined, weighted via a temperature-dependent
characteristic curve, into the overall result. The values emerging
from FIG. 6 are composed as follows: [0065] NH.sub.3 MET: metered
NH.sub.3 (signal from the metering device) [0066] NH.sub.3CONV:
converted NH.sub.3 [0067] NO.sub.XV: NO.sub.x concentration
upstream of SCR catalytic converter (signal from characteristic
diagram, calculation or sensor) [0068] NO.sub.XV[sic; NO.sub.XN]:
NO.sub.x concentration downstream of SCR catalytic converter (NOx
sensor) [0069] TSCR: SCR catalytic converter temperature [0070]
NH.sub.3 ST.sub.--.sub.Balance: stored NH.sub.3 from balance [0071]
NH.sub.3ST.sub.--.sub.charact. diag.: stored NH.sub.3 from
characteristic diagram [0072] NH.sub.3ST.sub.--.sub.act: stored
NH.sub.3 [0073] ETA.sub.SCR: efficiency of the SCR catalytic
converter [0074] NO.sub.2/NO.sub.x: ratio of NO.sub.2 to NO.sub.x
concentration upstream of SCR catalytic converter [0075] RG:
spatial velocity
[0076] FIG. 7 shows in an upper representation that a rapid rise of
the SCR catalytic converter temperature can occur in the case of a
sharp increase in the load. In comparison to this, an increased
load at the same time is indicated by a dotted line in the lower
representation. Due to the elevated temperature, a higher formation
of NO.sub.x occurs, and at the same time, there is a decrease of
the storage capability of NH.sub.3 in the SCR catalytic converter.
This is indicated by the dashed curve, which indicates the maximum
NH.sub.3 that can be stored, while the currently stored NH.sub.3 is
indicated by the dot-dash line. Due to the increased load and the
temperature increase generated thereby, there can be the effect
that, even with metering deactivated, the amount already stored in
the SCR catalytic converter can no longer be completely reacted in
the form of NO.sub.x conversion. This causes the ammonia to escape
into the environment when the currently stored NH.sub.3 value and
the maximum storable NH.sub.3 value intersect. This is illustrated
by the NH.sub.3 slip that is shown. The operating mode of the
internal combustion engine shown thereunder indicates that the
temperature of the SCR catalytic conversion also rises in case of
an increased load. Therefore the maximum storable NH.sub.3 also
declines here. By changing the operating mode of the internal
combustion engine, however, a higher NH.sub.3 value can be made
available. If in addition to a higher raw NOx emission, a lower
fuel consumption is achieved, for example, by means of a reduced
exhaust gas return rate or an advanced beginning of injection, the
slip formation can be countered. As shown, the currently stored
NH.sub.3 level decreases in such a manner that it remains below the
maximum storable NH.sub.3 limit value. As shown above, the maximum
storable NH.sub.3 value can also be used as a limit value in order
to check to what extent the regulation and, in particular, a load
changeover actually is functioning. For example, monitoring can be
assured in this case by a sensor recording of a possible NH.sub.3
slip.
* * * * *